1. Field of the Invention
The present invention relates to a method and to an apparatus for modifying addresses of a memory control of a one-chip microcomputer having an external expandable memory, whereby an internal memory having a storage capacity of n binary words is provided in the microcomputer, and whereby the addressing of the external memory is carried out by way of a predetermined plurality m of connections at the microcomputer, and whereby the microcomputer delivers the addresses, both for the internal memory and the external memory, successively, beginning from a smallest address value, and whereby a transfer from the addressing of the internal memory to the addressing of the external memory follows in the microcomputer for addresses beyond the address range for the internal memory.
2. Description of the Prior Art
In one-chip microcomputers, further components, in particular a program memory, are also integrated in a semiconductor chip, in addition to the central processor (microprocessor). It has also become known from the German published application No. 2,654,278 to expand the storage capacity in one-chip microcomputers by means of a memory that can be externally connected to the microcomputer. With this arrangement, for addressing of the memory, a possibility is provided for transferring, during addressing, from the internal (co-integrated) memory to the external memory. A specific plurality of externally accessible connections is required in the microcomputer for the addressing of the external memory, as well as for data traffic.
In integrated circuits, the expense for housing of the components, including the connections leading to the exterior of the device, generally represents a significant factor which must be taken into consideration in comparison to the expense for the circuit system, i.e. the circuit without the electrical connections leading to the outside and without housing. Within the conception of integrated circuits, therefore, one generally endeavores to keep the number of connections on the housing, and thus the expense for the housing, as low as possible. Particularly in microprocessors, and in microcomputers, it can therefore be expedient to undertake additional circuit technical expense in the circuit system in order to eliminate connections on the housing, since additional circuit components in the circuit are of relatively less consequence, in comparison to additional connections on the housing.
In a one-chip microcomputer having externally expandable memory capacity, therefore, the memory capacity of the external memory and, therefore, the total memory capacity realizable by means of the internal and external memories is limited by the number of connections on the housing which one wishes to allow in toto for the address and data traffic between the microcomputer and the external memory.
In a one-chip microcomputer, according to the aforementioned German published application No. 2,654,278, the arrangement is specifically laid out in such a manner that a 1 Kbyte read only memory (ROM) is located on the microcomputer chip and is externally expandable to a maximum of 4 Kbytes. Further, the arrangement is laid out in such a manner that, given a value range of 1-1023 for the internal memory, addresses which exceed the value of 1023 are delivered, essentially unaltered, to the external memory via 12 externally accessible connections on the microcomputer.
If one proceeds generally from the fact that the internal memory has a memory capacity of n words and that m connections for addressing the external memory are available at the microcomputer, then a total of 2.sup.m words can be addressed in the type of addressing mentioned above for the total memory formed of the internal and external memories in the known microcomputer. The usable memory capacity of the external memory which is available is directly connected with the plurality of connections on the microcomputer and can be expanded only by increasing the plurality of such connections.